1. Field of the Invention
The invention relates in general to on-screen display (OSD) and in particular to a device that performs complex, overlaying of characters, graphics, etc. utilizing a small-scale display scan memory.
2. Prior Art
Conventional OSD devices have generally been designed to first store in display memory simple object characters, their colors and other very basic attributes. These OSD devices then read the display data from the display memory, according to incoming horizontal and vertical synchronizing pulses, retrieve corresponding font data from a character generator ROM and finally display the characters with the specified colors and other attributes. These steps require specialized hardware OSD circuitry.
A more recent architecture is based on a bit map display system, where complex OSD is made possible by allocating each pixel multiple bits in a large-scale, external, video display memory, that is controlled by a dedicated, graphics controller integrated circuit.
The extensive use of hardware circuitry in the conventional OSD devices has not only inherently limited their complex OSD capabilities, but have made them much less flexible, requiring extensive redesigning every time improvements are called for. The bit map display system, while allowing complex OSD, make their host products costly because they require a large-scale, external memory and a dedicated integrated circuit for fast display data generation.